In a contamination inspection device, a spatial filter comprised of liquid crystal filters on which optional patterns are formed by picture elements consisting of dot matrices, forms a light shading pattern of diffracted lights projected on a Fourier transformation surface, by tuning on or off each picture element (that is, penetrating or shading lights). This may provide the light shading pattern formed by the spatial filter excellent in diversity of the pattern. However, incomplete light shading ability of a liquid crystal filter may cause malfunction allowing diffracted lights projected on a Fourier transformation surface, which must be essentially shaded, to penetrate the liquid crystal filters. In other words, this malfunction results in the lowering of the detecting accuracy. Therefore, in order to solve the above mentioned malfunction, a product improved with a shading rate thereof has been proposed, formed by stacking a plurality of liquid crystal filters. For example, Japanese Unexamined Patent Application Publication No. H11-352075 (or Patent Document 1) discloses the above mentioned invention.
Further, there is a mechanical spatial filter composed of light shading materials and coil springs, which is configured such that the light shading materials are joined to the coil springs. Such a mechanical spatial filter allows an interval between the light shading materials to be controlled and the light shading materials to be arranged at equal intervals through expanding or contracting the coil springs. Accordingly, the mechanical spatial filter is effective in an inspection target having a regular pattern. For example, Japanese Unexamined Patent Application Publication No. H10-68698 (or Patent Document 2) discloses the above mentioned invention.